The receptor-related protein (RAP) competes with all known ligands of the low density lipoprotein receptor family, particularly the low density receptor-related protein (LRP) or the ?2-macroglobulin receptor. Initially, it was thought that RAP might function as a modulator of cell surface receptors; however, there is strong evidence that this protein likely functions in receptor folding and/or trafficking, serving as a new class of chaperones. The protein was crystallized from ammonium phosphate solutions as thin plates with a tendency to form clusters or twinned crystals. This unfavorable morphology does not allow us to use a standard approach x-ray investigation. Attempts to improve the morphology of the crystals were unsuccessful. Synchrotron intense x-ray beam is the only means to collect data from these thin crystals.